In the early days of Radar, it was discovered that thin pieces of aluminium have a high radar reflectivity. This property was used during the World War II and was called “Chaff”. An aircraft spread a cloud of small, thin pieces of for example aluminium, which either appears as a cluster of secondary targets on radar screens or swamps the screen with multiple returns. Modern armed forces use chaff, for example in naval applications, using short-range SRBOC rockets, to distract radar-guided missiles from their targets. Most military aircraft and warships have chaff dispensing systems for self-defense. The length of the chaff should be approximately half the wavelength of the radar.
The inventor of the present invention found that a metal string, which was caused to vibrate with its fundamental frequency, imposed an amplitude modulation on a microwave radiation, such as a radar signal. Thus, WO 01/73389 discloses a metal string, which is caused to vibrate. The frequency of the vibrations is dependent on the length, density and tension of the string. The vibration frequency is also influenced upon by temperature. A microwave transmitter directs an electromagnetic microwave signal towards the string and the reflections there from are received by a microwave receiver. The received signal is amplitude modulated by the frequency of the vibrating string. Thus, for example temperature can be measured from a distance. By connecting the string to a pressure membrane, so that the pressure influences upon the tension of the string and thus on the vibration frequency, the pressure can be monitored from a distance. Force and torque can also be measured indirectly.
U.S. Pat. No. 6,492,933 discloses a microwave sensor that employs single sideband Doppler techniques in innumerable vibration, motion, and displacement applications. When combined with an active reflector, the sensor provides accurate range and material thickness measurements even in cluttered environments. The active reflector can also be used to transmit multi-channel data to the sensor. The sensor is a homodyne pulse Doppler radar with phasing-type Doppler sideband demodulation having a 4-decade baseband frequency range. Ranging is accomplished by comparing the phase of the Doppler sidebands when phase modulated by an active reflector. The active reflector employs a switch or modulator connected to an antenna or other reflector. In one mode, the active reflector is quadrature modulated to provide SSB reflections. Applications for the low-cost system include a mechanical motion/rotation sensor, a robust security alarm, a throat microphone, a stereo guitar pickup, a direction sensitive cardiac monitor, an electronic dipstick, a material thickness/dielectric sensor, a metal smoothness meter, a non-contact electronic readout, an RFID tag, silent “talking” toys, a passive-emitter data link, a beam interrupter, and a gold nugget finder.
These previously known devices operate well in many applications. However, in harsh environment and at large distances, the received signal is weak and cluttered by reflexes from other objects. Thus, it may be difficult to discriminate the signal from the noise floor.
There is a need in the art for a microwave device for sensing a vibrating object with an improved signal-to-noise ratio.